Abstract. The media required for micro-organisms to grow and reproduce are formulated based on the nutrient concentration in the natural environment in which they thrive. Nutrients are required for cyanobacteria to form carbohydrates, proteins, lipids and nucleic acids. These organisms attract the attention of researchers because they can extract biofuel from them. For most non photosynthetic micro-organisms, the nutrients present in the media serves as the energy and carbon source. But cyanobacteria are photoautotroph and light serves as the energy source and CO2 is the carbon source. The nutrients in the media are necessary for developing vital organelles, cell division, photosynthesis and growth.

Different nutrients are supplied in different quantities based on the requirement of the cells. The limiting nutrient is the one which is completely used up by the cells first thereby limiting cell growth and cell division. The wild strain Anabaena 7120 is grown in BG-11 media. Three major nutrients nitrate, phosphorus and sulfate are measured in the media to determine the decrease in quantity of the nutrient, the change in quantity is assumed to be taken up by the cell. These nutrients are quantified using a spectrophotometer. The relationship between biomass dry weight and optical density was also obtained.

The results show that the nutrient nitrate was consumed by the cell up to certain level and then it increased in the media. Sulfate concentration remained the same and the amount of phosphorus gradually decreased, to zero. Thus the nutrient phosphorus is the limiting nutrient for the strain Anabaena 7120 wild type grown in BG-11 media.